Key holder for an optical key and system comprising the key holder for authenticating an optical key by verifying a match of challenge-response pairs

ABSTRACT

A key holder includes a ferrule, a multimode light guide at least partly embedded inside the ferrule, an optical key which has a light scattering material, and a mechanical mount which mounts each of the ferrule, the multimode light guide, and the optical key. The multimode light guide has a front facet and a back facet which are arranged at opposite ends. The back facet of the multimode light guide contacts the optical key. Light can enter into the multimode light guide via the front facet, propagate through the multimode light guide, be scattered by the optical key, and propagate back through the multimode light guide and exit via the front facet. The mechanical mount is detachably connected to a mechanical mount terminator. The front facet of the multimode light guide is oriented in a direction of the mechanical mount terminator.

CROSS REFERENCE TO PRIOR APPLICATIONS

This application is a U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2021/025488, filed on Dec.10, 2021 and which claims benefit to U.S. Provisional Patent ApplicationNo. 63/124,135, filed on Dec. 11, 2020. The International Applicationwas published in English on Jun. 16, 2022 as WO 2022/122185 A1 under PCTArticle 21(2).

FIELD

The present invention is related to the field of Physical UnclonableKeys (PUKs), in particular to optical PUKs (optical keys). The presentinvention is more particularly related to a key holder comprising theoptical key and to a system comprising the key holder. The system can,for example, be a system for authenticating an optical key by verifyinga match of challenge-response pairs.

BACKGROUND

Secure communication and authorization are highly important topic thesedays. Many secure communication and authorization schemes rely oncryptographic systems and methods. In classical cryptography, themathematically (still not entirely proven) fact is used that somemathematical functions can be calculated in one direction without anyproblems, but solving the inverse mathematical problem is hardlypossible; at least this would require extremely high and time-consumingcomputational efforts. However, these computational efforts willsuddenly be significantly reduced when the first full-scale quantumcomputers will start operation. A search therefore exists for otherapproaches which are inherently secure.

In practical cryptography, a physically unclonable function or PUF is afunction that is embodied in a physical structure and is easy toevaluate, but hard to predict and assumed to be physically unclonablebecause of a strong dependence on uncontrollable aspects of themanufacturing process. PUFs have a unique challenge-response behavior.For these reasons, they are of interest as means of authentication. Inthis context, the term “physically unclonable key” (PUK) is used as asynonym for the term PUF.

Typically, a PUK owner should prove access to a secret by presenting hisPUK to a verifying party. The verifying party sends a signal calledchallenge to the PUK, and the PUK then creates a unique and hard topredict reply signal which is called a response. This response issupplied back to the verifying party so that it can be verified that thePUK owner actually has authorized access to the secret or resource.

An example for a PUK is an optical PUK or optical key, the twoexpressions being used as synonyms within the present application. ThePUK can in particular act on the basis of light scattering, either intransmission or in reflection operation mode. The PUK can, for example,be a pigment such as ZnO or TiO₂ provided on glass, or more stably inglass or glass ceramics (glass containing nanoparticles) or in PMMA(like a DVD), or ceramics or glass ceramics themselves. Another exampleis biologic material which has the potential to authenticate people bythe unique properties of their body. Such unique biometric data can, forexample, be teeth, bones or even parts of the human eye. These materialsfulfill a general requirement for PUK interaction, namely, that thematerial of the PUK/the scattering media included in the PUK have ascattering mean-free path which is short. A rough surface alone cannotbe used as a PUK, mostly because a surface geometry can be copiedrelatively easily, not because a surface is not complex.

EP 2 693 685 B1 describes a quantum secure device, system and method forverifying challenge-response pairs using an optical key, the disclosureof which is incorporated by reference herein in its entirety. EP 2 693685 B1 describes wavefront shaping techniques which are applied tocompensate for the PUK's light scattering behavior. A correctlycompensated wavefront will be transformed back into a plane wave bywavefront shaping. This plane wavefront allows for focusing the receivedlight and detecting a validation signal. Any other wavefront will createa random speckle pattern, no focusing is possible, and the validationfails. The disclosed device, system and method are also quantum secure.The system can in principle also be applied in the classical regime, itssecurity then relying on strong statistics or the speed at which theresponse is provided, preventing digital emulation attacks.

A critical component for bringing the known authentication schemes likeQSA (quantum-secure authentication) into practice is the optical key orPUK. Choosing an optical set-up component that can be used as a PUK isnot trivial. It should have a very high positioning precision. This isrequired since the PUK's structure strongly varies over very smalldistances. Light incident on the PUK will consequently scatter verydifferently if the PUK has moved only slightly with respect to theincident light. If a PUK is to be reliably and repeatably used inauthentication protocols, positioning precision in the order of thewavelength of the used light is required for the PUK.

In first demonstrator set-ups, the position of the PUK was permanentlyfixed with respect to the other optical components. In first steps forrealizing a PUK that can be repeatably used in authentication protocols,a so-called kinematic base was used for holding the PUK. A kinematicbase is a standard device in optics for mounting elements that need tobe inserted and removed from the optical path with a high degree ofrepeatability and precision. Kinematic bases are commercially availableand are manufactured and purchased by specialized firms (for example,Thorlabs®). A kinematic base basically comprises a top plate and abottom plate that are securely coupled together by a sophisticatedmechanism that allows for highly precise positioning and in particularrepositioning. The coupling between the two plates normally worksmagnetically or mechanically.

In a respective demonstrator's set-up, the PUK material was permanentlyfixed (glued) to the removable plate (top plate) of the kinematic base.However, it surprisingly turned out in numerous experiments that thepositional precision that could be achieved when putting the top plateback into place was not good enough for the intended purpose of PUKauthentication.

SUMMARY

An aspect of the present invention is to provide a key holder for anoptical key that allows for positioning an optical key with higherprecision where a highly precise positioning is repeatedly possible. Thekey holder must furthermore be easy to manufacture and withcomparatively low cost.

In an embodiment, the present invention provides a key holder whichincludes a ferrule, a multimode light guide at least partly embeddedinside the ferrule, an optical key which comprises a light scatteringmaterial, and a mechanical mount which is configured to mount each ofthe ferrule, the multimode light guide, and the optical key. Themultimode light guide comprises a front facet and a back facet which arearranged at opposite ends of the multimode light guide. The back facetof the multimode light guide contacts the optical key. The multimodelight guide is configured so that light can enter into the multimodelight guide via the front facet, propagate through the multimode lightguide, be scattered by the optical key, and propagate back through themultimode light guide and exit via the front facet. The mechanical mountis further configured to be detachably connected to a mechanical mountterminator. The front facet of the multimode light guide is oriented ina direction of the mechanical mount terminator.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in greater detail below on the basisof embodiments and of the drawings in which:

FIG. 1 schematically illustrates a set-up of a QSA demonstratoraccording to the state of the art, wherein a kinematic base is used as akey holder;

FIG. 2 schematically illustrates a sectional view of a system comprisinga key holder with an optical key and a mechanical mount terminatoraccording to a first embodiment of the present invention when the keyholder is demounted;

FIG. 3 schematically illustrates a sectional view of the system shown inFIG. 2 when the key holder with the optical key is mounted;

FIG. 4 schematically illustrates a second embodiment of the inventionwith a different optical key;

FIG. 5 schematically illustrates a third embodiment of the key holder;

FIG. 6 schematically illustrates a fourth embodiment of the key holder;and

FIG. 7 schematically illustrates a system for authenticating an opticalkey by verifying a match of a challenge-response pair according to theinvention.

DETAILED DESCRIPTION

Facing the problem that known highly precise mounting mechanisms inoptics are not suited as key holders for optical keys, the presentinvention provides combining mounting mechanisms which are already knownfrom other technical fields with the specific requirements for a PUKmount. Fiber connectors already routinely achieve sub wavelengthmounting accuracy. Fiber connectors have, however, to date only beenused as devices for joining optical fibers. The present invention hastherefore adapted known structures and characteristics of fibers and offiber connectors for a key holder for an optical key/optical PUK (thetwo terms being used as synonyms within the present application).

In more detail, according to a first aspect of the present invention,the present invention is directed to a key holder comprising:

a ferrule;

a multimode fiber piece at least partly embedded inside the ferrule;

an optical key comprising a light scattering material; and

a mechanical mount mounting the ferrule, the multimode fiber piece andthe optical key,

wherein the multimode light guide, in particular a multimode fiberpiece, comprises a front facet and a back facet provided at oppositeends of the multimode light guide and wherein the back facet of themultimode light guide contacts the optical key,

wherein the multimode fiber piece is adapted so that light can enterinto the multimode fiber piece via the front facet, propagate throughthe multimode light guide in particular via total internal reflection,be scattered by the optical key, and propagate back through themultimode light guide and exit via the front facet, and

wherein the mechanical mount is adapted to be detachably connected to amechanical mount terminator, wherein the front facet of the multimodelight guide is oriented in the direction of the mechanical mountterminator.

Light guides are optically transparent components such as fibers, tubesor rods that transport light over short or long distances. The light isconducted by reflection at the boundary surface of the light guideeither by total reflection due to a lower refractive index of the mediumsurrounding the light guide or by mirroring at the boundary surface orby a suitable refractive gradient.

A multimode light guide according to the present invention can thereforebe realized in different ways. A multimode light guide has theproperties of: a) guiding light, and of, b) transporting a plurality oftransverse spatial modes of light. Exemplary embodiments are therefore amultimode fiber, but also a bundle of single mode fibers. Other lightguides that fulfil the above requirements a) and b) are furthermore alsopossible realizations.

The shape of a multimode light guide is in principle not restricted. Inmost cases, the multimode light guide has an elongated shape in thegeneral direction of light transport, however, this can also bedifferent. A cross section of the multimode light guide can, forexample, be a circular cross section (which is the most common crosssection for fibers) or a rectangular cross section or it can haveanother shape.

According to the present invention, a combination of a multimode lightguide, in particular a multimode fiber piece, and an optical key arecombined in a key holder for the very first time. The optical keycontacting the back facet of the multimode light guide and thedimensions chosen for the multimode light guide allow the realization ofthe light propagation mechanism needed for a successful readout of theoptical key. According to the present invention, light can enter intothe multimode light guide, in particular into the multimode fiber piece,via the front facet, propagate through the multimode light guide, inparticular via total internal reflection, be scattered by the opticalkey, and propagate back through the multimode light guide and exit againvia the front facet. This light propagation must be possible for asignificant amount of light. In other words, the light intensity of thelight exiting via the front facet must not be too low, in particular ifthe key holder with the optical key is applied in a system forauthenticating an optical key by verifying a match of challenge-responsepairs in a quantum secure way. The light power entering the front facetis typically in the range of one or a few photons per challenge pulse(in a quantum secure approach, for example, with 200 photons) up to 100mW, and the light power exiting via the front facet again is in therange of one or a few photons per pulse up to 100 mW. The efficiencyq=Int_(exit)/Int_(enter) must be comparatively high, for example, q≥50%,for example, q≥70%.

In an embodiment of the present invention, the multimode light guidecan, for example, be a multimode fiber, or more precisely, a multimodefiber piece. Light normally propagates through a multimode fiber viatotal internal reflection. Total internal reflection within a fiber ispossible for light meeting the core-cladding boundary of the fiber pieceat an angle (measured relative to a line normal to the boundary) greaterthan the critical angle for this boundary. This critical angle isdetermined by the difference in index of refraction between the core andcladding materials. Rays that meet the boundary at a low angle arerefracted from the core into the cladding where they terminate. Thecritical angle determines the acceptance angle of the fiber, oftenreported as the numerical aperture NA of the fiber. More precisely, thenumerical aperture NA of a fiber is the sine of the critical angle of anincident beam. It is noted that light is focused to the front facet ofthe fiber for being coupled in. A high numerical aperture NA allowslight to propagate down the fiber in paths (geometric optics; rays) bothclose to the axis and at the various angles, allowing efficient couplingof light into the fiber.

Because light must strike the boundary of the fiber with an anglegreater than the critical angle, only light that enters the fiber withina certain range of angles can travel down the fiber without leaking out.This range of angles is called the acceptance cone of the fiber. Thismust be taken into consideration when combining the fiber with theoptical key/optical key material at the back facet of the multimodefiber piece. The scattering of light back into the fiber must basicallybe considered as a new entrance condition of light so that light cansuccessfully propagate back through the fiber.

According to the present invention, light propagating through themultimode fiber is scattered by the optical key being in contact withthe back facet of the multimode fiber. Due to the scattering process,the direction of light differs significantly from ray to ray/photon tophoton, and as a result, when scattered back, light re-enters themultimode fiber and propagates back through the multimode fiber in manydifferent angles. Due to the scattering, the angle with which rays orphotons meet the boundary of the fiber can also be comparatively small.According to an embodiment of the present invention, the numericalaperture NA of the multimode fiber can, for example, be chosen to becomparatively high. The numerical aperture NA of conventional opticalfibers is typically in the range of 0.2 to 0.3 at maximum. It has turnedout that this conventional range is not big enough for the purposes ofthe present invention. The following relation holds for the numericalaperture NA of the multimode fiber according to the present invention:NA≥0.50, for example, NA≥0.66. In principle, the numerical aperture NAcan, for example, be realized as high as possible. A numerical apertureNA≥0.70 is, however, rare and difficult to obtain commercially and mightlead to reflection or collection losses at the input facet. In anembodiment of the present invention, the following relation can, forexample, hold: 0.70≥NA≥0.50, for example, 0.70≥NA≥0.66.

The concrete kind of contact between the multimode fiber/multimode lightguide and the optical key can also be realized in different ways.Importantly, the back facet of the multimode light guide contacts theoptical key, for example, the optical key contacts the complete backfacet of the multimode light guide. It is also possible that the opticalkey is not only in contact with the back facet of the multimode lightguide, but that the optical key also surrounds an end part or a regionof the multimode light guide neighbored to the back facet. In thesecircumstances, the multimode light guide, more precisely one of itsends, is embedded into the optical key/the optical key material. It isnoted that the term “contact” is here used in the optical sense, so acontact can also be realized with a material (for example, epoxy) havingapproximately the same refractive index as the fiber core.

According to the present invention, a multimode light guide is combinedwith the optical key in the key holder. Using a multimode light guide isnecessary because challenge-response pairs are encoded in thespatial-frequency-domain. It is required that the transmission of thechallenge to the optical key/PUK takes place via an optical channelcontaining many transverse spatial modes.

In an embodiment of the present invention, the following relation can,for example, hold for a diameter d of a core of the multimode lightguide or the multimode fiber: d≥80 μm, for example, d≥100 μm. This isquite large for the diameter of a multimode fiber. However, this allowsthe use of a great number of transverse spatial modes inside the fiber.Typically, for authentication purposes with lots of light (for example afew mW), more than 10000 modes in a fiber can, for example, be used. Thediameter d and the number of modes in a fiber are inherently coupled. Ina quantum secure authentication procedure, the number of photons can,for example, be far below the number of modes. For example, a few 1000modes are controlled, but the number of photons is approximately 200.

The present invention provides that the multimode light guide is atleast partly embedded inside the ferrule. For example, a part of themultimode light guide embedded inside the ferrule can be glued into theferrule, for example, by using epoxy. Parts of the multimode light guidenot embedded inside the ferrule can be fully or at least partly incontact or embedded by the optical key, or more precisely, by theoptical key material. The optical key itself can have a defined shape,for example, it can be provided as a cylinder or a plug. It is alsopossible, however, that the optical key has no fixedly defined shape,but is just embodied as a material just contacting orsurrounding/embedding the back facet of the multimode light guide. The“shape” of such an optical key can also be defined by surroundingconditions, for example, the provision of another mechanical part in thekey holder.

The shape of the ferrule can, for example, be rotationally symmetric; inother words, the ferrule can, for example, have a standard shape and itscross section can, for example, be a circular ring. It is thereforepossible to use existing standard ferrules for realizations of thepresent invention. However, the shape of the ferrule, in particular itsinner and/or outer cross-sectional shape, can in principle also bedifferent, for example, rectangular.

According to the present invention, the key holder comprises amechanical mount for mounting the ferrule, the multimode fiber piece andthe optical key. The mechanical mount can here mount the ferrule, themultimode light guide and the optical key directly or indirectly. In anembodiment of the present invention, the mechanical mount can, forexample, directly mount the ferrule and the optical key, but not themultimode light guide which is provided inside the ferrule and theoptical key material. Direct mounting here means direct contact of thepart to be mounted and the mount. Indirect mounting means that anothermaterial or part can be inserted between the part to be mounted and themount. The mechanical mount itself can furthermore comprise a pluralityof parts.

According to the present invention, the mechanical mount is adapted tobe detachably connected to a mechanical mount terminator, wherein thefront facet of the multimode light guide is oriented in the direction ofthe mechanical mount terminator. The mechanical mount terminator is notpart of the key holder. The mechanical mount of the key holder and themechanical mount terminator are, however, both constructed to fittogether. With respect to the intended use for authentication purposesof the key holder with the optical key, it is important that the frontfacet of the multimode light guide is oriented in the direction of themechanical mount terminator when the mechanical mount is connected tothe mechanical mount terminator. Light probing the optical key entersinto the multimode light guide through the front facet and also exitsvia the front facet after having been scattered by the optical key. Thefront facet of the multimode light guide must therefore be oriented inthe direction of the mechanical mount terminator.

According to the present invention, the mechanical mount is adapted tobe detachably connected to a mechanical mount terminator. This meansthat the key holder with the mechanical mount can be detached withoutany damage from the mechanical mount terminator. It is instead intendedto repeatedly connect and disconnect the mechanical mount and themechanical mount terminator.

In an embodiment of the present invention, the mechanical mount can, forexample, be adapted to be connected to a mechanical mount terminator bya click mechanism or a screw mechanism. Both kinds of mechanism are inprinciple known from fiber connectors according to the state of the art.It is alternatively possible that the mechanical mount is adapted to beconnected to a mechanical mount terminator by a magnetic mechanism. Thestate of the art describes different kinds of fiber connectors whichhave proven that a highly precise connection mechanism in the form of aclick mechanism or a screw mechanism work: LC-fiber connectors andSC-fiber connectors apply a click mechanism when connected to acorresponding fiber connector terminator. FC-fiber connectors andST-fiber connectors are provided with a screw mechanism when connectedto the respective fiber connector terminators. These known principlescan be transferred to the key holder and the mechanical mount accordingto the present invention.

In an embodiment of the present invention, the mechanical mount can, forexample, comprise a rotation stop mechanism for enhancing the positionalaccuracy when connecting the mechanical mount to a mechanical mountterminator. If a connection between two parts of a fiber is only neededfor a secure transfer/transport of light, a rotation of the two partswith respect to one another normally does not have a negative influenceper se. This is different, however, when it comes to the use of fibersfor the purposes of authentication applying an optical key. Since theindividual scattering pattern of the optical key is here essential for asuccessful authorization, a rotation of the optical key material withrespect to any detection means outside the key holder must be avoided.The rotation stop mechanism can be provided by a single part or byseveral parts. It is noted that a rotation stop mechanism as such isalready known from special fiber connectors which are used when thepolarization of light must be kept constant during a light transfer/alight coupling process. Concepts known from these special fiberconnectors can be transferred to the present invention.

In an embodiment of the present invention, the rotation stop mechanismcan, for example, comprise a guiding ring coaxially provided with theferrule and outside the ferrule, the guiding ring being adapted to beslidably guided inside or outside around the corresponding terminatorguiding ring provided at the mechanical mount terminator, the guidingring comprising at least one radially protruding pin for securing themechanical mount against rotation in the mechanical mount terminator.The mechanical mount terminator can, for example, be provided with arespective bore hole which accepts the protruding pin of the mechanicalmount. The guiding ring of the key holder and the terminator guidingring are constructed to fit one another in a sliding way so that oneguiding ring contacts the other guiding ring of the outer face or on theinner face. The order of the rings in the radial direction does not inprinciple matter, both orders are possible. The radially protruding ringcan consequently be radially protruding to the outside or to the insideas well (positive and negative radial direction).

In an embodiment of the present invention, the mechanical mount can, forexample, comprise a push mechanism, for example, a spring mechanism, forpushing the multimode light guide towards a mechanical mount terminatorwhen connecting the mechanical mount to a mechanical mount terminator.This allows for a better connection between the mechanical mount and themechanical mount terminator and therefore for a tighter and more precisepositioning of the front facet of the multimode light guide throughwhich light exits the key holder again.

The spring mechanism can in principle be embodied in one part or by aplurality of parts. Spring mechanisms as such are known in the art.

In an embodiment of the present invention, the mechanical mount can, forexample, comprise a ferrule mount that is basically provided in the formof a hollow cylinder, wherein an end part of the ferrule is providedinside the hollow cylinder and wherein at least a part of the opticalkey is also provided inside the hollow cylinder, the end part of theferrule and the part of the optical key being connected to the ferrulemount, respectively. The back facet of the multimode light guide can,for example, therefore provided inside the ferrule mount, more preciselyinside the hollow cylinder of the ferrule mount. It is, for example,possible that the ferrule and the part of the optical key are glued tothe hollow cylinder of the ferrule mount. Other kinds of connections arealso, however, possible. The ferrule can for instance also be screwed orclamped into the hollow cylinder. The optical key can, for example, alsoembed an end part of the multimode light guide and not only contact theback facet of the multimode light guide, exclusively.

In an embodiment of the present invention, the optical key can, forexample, be provided completely inside the ferrule. Parts of the ferrulecan therefore surround or be filled with the multimode light guide orthe optical key. It is also, however, possible that a remaining part ofthe ferrule which is arranged more distant to the front facet of themultimode light guide than the optical key be filled with a filling(shape stabilizing) material or that this remaining part even be empty.Referring back to the previously described embodiment, it is thenpossible that the mechanical mount comprises a ferrule mount that isbasically provided in the form of a hollow cylinder, wherein an end partof the ferrule is provided inside the hollow cylinder and is connectedto the ferrule mount. The optical key can also be provided inside thehollow cylinder, but it can be also arranged outside the hollowcylinder, depending on the length of the multimode light guide. Themultimode light guide and the optical key together in principle have alength in the direction of the axis of the ferrule that is equal to orshorter than the length of the ferrule in said direction. It is alsopossible that the multimode light guide is extremely short or that themultimode light guide be completely omitted. It can then be said thatthe optical key and the multimode light guide are combined into oneelement. The optical key is as a result then provided directly at thefront facet of the ferrule.

According to a preferred embodiment of the present invention, theferrule mount is provided inside a casing of basically cylindricalshape, wherein a spring mechanism coupled to the ferrule mount and thecasing is provided inside the casing to push the towards a mechanicalmount terminator. The casing is part of the mechanical mount as well. Bypushing the ferrule mount towards a mechanical mount terminator, thefront facet of the multimode light guide is pushed towards themechanical mount terminator as well since the multimode light guide ismounted to the ferrule mount. This measure contributes to more precisepositioning of the key holder with respect to the mechanical mountterminator and a detector system for validating the optical key.

In an embodiment of the present invention, a spatial positional accuracyof the mechanical mount with respect to a mechanical mount terminatorcan, for example, be is equal to or better than 1 μm in each of thethree spatial directions and/or and angular positioning accuracy of themechanical mount with respect to a mechanical mount terminator can, forexample, be equal to or better than 1° in each of the three angulardirections. The three spatial directions and the three angulardirections can here in principle be freely chosen. It makes sense inpractice, however, to define the three spatial directions as thedirections of the three principal axes of the key holder, and to definethe three angular directions as rotations around each of these principalaxes. The precision mentioned above can be realized with one or more ofthe measures described above in more detail. If the accuracies arebetter that 1 μm, this accuracy is in the order of magnitude of thewavelength typically applied for authentication purposes.

As already described in the introductory part of the presentapplication, different materials can in principle be used as a part ofor as a PUK material. The material of the PUK/the scattering mediaincluded in the PUK have a scattering mean-fee path which is short andwell controlled. In an embodiment of the present invention, the opticalkey comprises a material mixture of epoxy and scattering particles, forexample, a mixture of epoxy and zinc oxide powder. Using a materialmixture of epoxy and scattering particles has the advantage that fixingthe multimode light guide to the ferrule and providing the optical keyon the back facet of the multimode light guide becomes easier in themanufacturing processes. Epoxy can be used to normally glue themultimode light guide to the ferrule. It can then also be used to gluethe scattering particles to the back facet of the multimode light guideas well as possibly to neighboring regions/the end part of the multimodelight guide. It is then easiest to mix the epoxy and the scatteringparticles and to provide this material mixture for connection purposes.For the epoxy or an alternative material into which the scatteringmaterials are embedded, it is also necessary that it is opticallytransparent and that its refractive index is, for example, rather closeto the refractive index of the light guide core or fiber core.

In an embodiment of the present invention, the optical key can, forexample, be provided as a plug, in particular a plug comprising a lightscattering ceramic. It is possible that the plug only contacts a backfacet of the multimode light guide, however, the plug can, for example,comprise an opening or a small borehole into which the multimode fibercan be inserted. The inserted part of the multimode light guide canadditionally be glued to the plug, for example, by epoxy.

In an embodiment of the present invention, the optical key can, forexample, comprises a ceramic material, in particular a glass ceramicmaterial with at least two different phases, wherein at least one of thephases defines a structure which determines the response of the opticalkey when receiving the challenge created for authentication purposes.Different kinds of ceramic materials that are suited as optical key arealready known. More detailed information also about the thermalreliability of optical keys is described in the EP 3 252 740 B1, theentirety of which is incorporated by reference herein.

The numerous embodiments of the key holder as described above can becombined with one another fully or in part as long as no technicalcontradictions occur.

According of a second aspect of the present invention, the presentinvention is directed to a system comprising:

the key holder as defined in any one of the embodiments as describedabove comprising the mechanical mount; and

a mechanical mount terminator,

wherein the mechanical mount is detachably mounted or detachablymountable to the mechanical mount terminator. The mechanical mountterminator will in practice be used in combination with a plurality ofkey holders since it is realistic that a plurality of people having aplurality of optical keys ask for authentication for the samesystem/secret.

In an embodiment of the present invention, the accuracy with which thekey holder is detachably mountable or detachably mounted to themechanical mount terminator can, for example, be equal to or better than1 μm. The angular positioning accuracy can, for example, be equal to orbetter than 1°, for example, equal to or better than 0.5°, for example,equal to or better than 0.1°. Concerning the details of the namedprecision, reference is made to the respective paragraph in thedescription of the key holder describing and further defining thespatial positioning accuracy and the angular positioning accuracy.

In an embodiment of the present invention, the system can, for example,further comprise an objective, wherein the mechanical mount terminatoris fixedly arranged with respect to the objective. The objective can,for example, be part of a system for authenticating an optical key byverifying a match of a challenge-response pair. The objective can,however, in principle be provided for other purposes or reasons. Theoptical axis of the objective can, for example, be aligned with theoptical axis of the mechanical mount terminator and also aligned withthe optical axis of the multimode light guide. This allows for an easieralignment of the different parts with respect to one another. Theobjective is used to focus light onto the front facet of the multimodelight guide, the light then entering into the multimode light guide.

In an embodiment of the present invention, the mechanical mountterminator can, for example, comprises a base plate having athrough-hole facing the front facet of the multimode light guide whenthe mechanical mount is mounted to the mechanical mount terminator.Light used for probing the optical key can therefore pass the mechanicalmount terminator without obstruction and can directly enter themultimode fiber piece.

In an embodiment of the present invention, the base plate can, forexample, further comprise a ring-shaped reception part which iscoaxially arranged with the optical axis of the system, the receptionpart being adapted for receiving at least part of the ferrule. Thedimensions of the ring-shaped reception part and of the outer face ofthe ferrule are adapted to one another. The ferrule can therefore beslidingly inserted into the ring-shaped reception part. The position ofthe ferrule is stabilized by the contact to the ring-shaped receptionpart.

In an embodiment of the present invention, the base plate can, forexample, further comprise a ring-shaped fixation part which is coaxiallyarranged with respect to the ring-shaped reception part and which isarranged radially outside and with a radial distance to the receptionpart, the ring-shaped fixation part comprising a bore hole for receivinga pin of the mechanical mount in order to secure the mechanical mountand the mechanical mount terminator against relative rotation. Themechanical mount terminator therefore basically comprises a base platewith a protruding double ring structure. The outer ring is thering-shaped fixation part, the inner ring is the ring-shaped receptionpart. In addition to its function as a rotation stop, the ring-shapedfixation part can, for example, be intended for fixing and in particularfor detachably fixing the fixation part to a corresponding fixation partof the mechanical mount. The fixation mechanism can, for example, be ascrew mechanism or a click mechanism as already described above. Thebore hole provided within the ring-shaped fixation part is orientedparallel to the optical axis of the system/the mechanical mountterminator.

The embodiments of the system as described above can be fully or partlycombined with one another as long as no technical contradictions occur.

According to a third aspect of the present invention, the presentinvention is directed to a system for authenticating an optical key byverifying a match of a challenge-response pair, comprising:

a challenge forming device for forming a challenge;

an optical key which comprises a scattering material and is receptive tothe challenge forming device;

a response verifying device which is receptive to the response providedby the optical key for verifying if the response provided by the opticalkey matches the challenge form by the challenge forming device,

characterized by the system as described above in numerous embodimentsaccording to the second aspect of the present invention, wherein theoptical key is held by the key holder. The key holder as such has beendescribed with respect to the first aspect of the present inventionaccording to several embodiments. The system to which the system forauthenticating an optical key by verifying a match of achallenge-response pair refers back is a simple system combining the keyholder as described above and the mechanical mount terminator, whereinthe mechanical mount is detachably mountable or detachably mounted tothe mechanical mount terminator.

Systems for authenticating an optical key by verifying a match of achallenge-response pair are in principle known, however, thecharacterizing feature according to the present invention is embodied bythe specific key holder which is mounted to the corresponding mechanicalmount terminator. With respect to systems for authenticating an opticalkey by verifying a match of a challenge-response pair, reference is madeto the EP 2 693 685 B1, the entirety of which is incorporated byreference herein, and to EP 3 252 740 B1, the entirety of which isincorporated by reference herein.

In an embodiment of the present invention, in the system forauthenticating an optical key by verifying a match of achallenge-response pair the following relation holds:

q=Int_(exit)/Int_(enter)≥50%, for example, q≥70%,

wherein Int_(enter) is the intensity of light entering the multimodelight guide via the front facet and Int_(exit) is the intensity of lightexiting the multimode light guide via the front facet after having beenscattered by the light scattering material of the optical key. The ratioq (light efficiency) provides that in particular for quantum securesystems, the light intensity exiting the front facet of the multimodelight guide is high enough that a response verification and/or detectionof the response or its validity can be successfully undertaken.

In an embodiment of the present invention, the present invention isdirected to a mechanical mount terminator which is adapted to fit to themechanical mount of the key holder as described above in variousembodiments. The mechanical mount terminator can furthermore have afeature or several features of the mechanical mount terminator furtherdescribed with respect to the system according to the second aspect ofthe invention, said system comprising the key holder with a mechanicalmount and the mechanical mount terminator, wherein the mechanical mountis detachably mountable or detachably mounted to the mechanical mountterminator.

The above-described aspects of the present invention can be combinedfully or in part with one another, the same holds for the numerousembodiments according to the numerous aspects of the present invention,as long as no technical contradictions occur.

The present invention is described in greater detail below underreference to the drawings where same reference signs indicate the samefeatures.

FIG. 1 schematically illustrates a set-up of a QSA demonstrator 1000according to the state of the art wherein a kinematic base 100 is usedas a key holder. The depicted set-up is just an example, other set-upsare also possible and are in principle known by the person skilled inthe art. A coherent light source, for example, a laser 106, emits laserpulses having a flat wavefront in the depicted example. The laser pulsewith a flat wavefront enters a DMD (digital micromirror device) 108, forexample, a spatial light modulator SLM which acts on the phase and/oramplitude of the flat wavefront and shapes the wavefront into a randomor irregular wavefront. The pulse with the random or irregular wavefrontrepresents the challenge. The propagation direction of the light isindicated by the arrows 115 shown in the drawing. Light exiting the DMD108 passes through a lens 109 and an aperture 110, and then passesthrough a beam splitter 111 and another lens 112. The light then passesthrough a partial beam splitter 113 and is focused by an objective 105onto an optical key 103 which is provided on a kinematic base 100. Inmore detail, the Fourier transform of the shaped wavefront is projectedonto the key, via the objective 105. This is because the optical key 103is positioned in the image plane of the aperture 110, which in turn ispositioned in the Fourier plane of the DMD 108.

Light that is scattered/reflected by the optical key 103 propagates backthrough the objective 105 and afterwards through the polarizing beamsplitter 113. The light then enters a camera or CCD 113. A half-waveplate 107 is additionally provided in the beam path. It is noted thatadditional optical components can also be provided within the beam path,the depicted drawing just being an illustration of the principles of theQSA demonstrator set-up 1000. If the challenge sent to the optical key103 and the response created by a scattering by the optical key 103match, this match can be detected by the CCD 114. In an embodiment, theresponse is a sharp focus at a particular spot out of a number(typically between 2 and 100) of possible spots. Generating only one ora few spots at preselected location(s) for verifying challenge-responsepairs has the advantage that the verification can be carried out withonly a few photons, and thus (also) quantum secure. Larger responsepatterns do not have this advantage.

Described differently, the QSA demonstrator set-up 1000 is an examplefor a system for authenticating an optical key 103 by verifying a matchof the challenge-response pair, comprising a challenge forming device108 for forming a challenge, an optical key 103 which comprises ascattering material and is receptive to the challenge formed by thechallenge forming device 108, a response verifying device 114 which isreceptive to the response provided by the optical key 103 for verifyingif the response provided by the optical key 103 matches the challengeformed by the challenge forming device 108. In the depicted example, theoptical key 103 is held by a key holder which is embodied as kinematicbase 100.

The kinematic base 100 is in principle built up as follows: Thekinematic base 100 comprises a top plate 101 and a bottom plate 102. Thetwo plates 101 and 102 are securely coupled together by a sophisticatedmechanism that can, for example, work mechanically or magnetically. Inthe depicted example, structures 104 provide exact positioning andrepositioning of the top plate 101 onto the bottom plate 102. The topplate 101 can be termed a key holder since it holds the optical key 103.In the depicted example, the PUK material is glued to the bottom plate101 and the PUK material 103 is provided in the focal plane of theobjective 105.

Despite the fact that kinematic bases like the kinematic base 100 arestandard devices in optics for mounting elements that need to beinserted and removed from the optical path with a high degree ofrepeatability and precision, it has turned out that the positioningprecision that could be achieved when putting the top plate 101 backinto place onto the bottom plate 102 was not good enough for theintended purpose of PUK authentication. Active alignment tools could betested whether they can achieve the necessary precision, however, theyare much more complex and hence, costly.

The present invention solves this technical problem by providing a keyholder for an optical key that allows for positioning an optical keywith higher precision. Highly precise positioning is in particularrepeatedly possible according to the present invention. It shallfurthermore be realized comparatively easy and with comparatively lowcost.

FIG. 2 schematically illustrates a sectional view of a system 80comprising a key holder 1 with an optical key 4 and a mechanical mountterminator 50 according to the present invention when the key holder 1is demounted. The key holder 1 comprises a ferrule 2 and a multimodelight guide 3 that is partly embedded inside the ferrule 2. In thedepicted embodiment, the multimode light guide is realized as amultimode fiber piece 3. Other realizations of the multimode light guide3 are also, however, possible in this and in the following embodiments.The key holder 1 further comprises an optical key 4 comprising a lightscattering material and a mechanical mount 5 mounting the ferrule 2, themultimode fiber piece 3, and the optical key 4. In the presentedexample, the ferrule 2 and the optical key 4 are directly mounted andare in direct contact with a ferrule mount 11, whereas, in contrastthereto, the multimode fiber piece 3 is only indirectly mounted since itis provided inside the ferrule 2 and inside the optical key material 4,respectively.

The multimode fiber piece 3 comprises a front facet 6 and back facet 7provided at opposite ends of the multimode fiber piece 3. The back facet7 of the multimode fiber piece 3 contacts the optical key 4. In thepresent example, the contact is not just at the back facet 7, but aregion of the multimode fiber piece 3 neighboring the back facet 7 isalso embedded inside the optical key 4 or the optical key material 4. Inthe present case, the optical key 4 comprises a material mixture ofepoxy and scattering particles, for example, a mixture of epoxy and zincoxide powder. A material can, for example, be used with low thermalexpansion properties, for example, F123 epoxy. Other materials can alsobe used as the PUK material.

According to the present invention, the multimode fiber piece 3 isadapted so that light can enter into the multimode fiber piece 3 via thefront facet 6, propagate through the multimode fiber piece 3 via totalinternal reflection, can be scattered by the optical key 4, propagateback through the multimode fiber piece 3, and exit the multimode fiberpiece 3 via the front facet 6 of the multimode fiber piece. In order toprovide this requirement, a fiber with a numerical aperture of 0.66 wasapplied in the present example, furthermore, the fiber core had adiameter d=100 μm. It is noted that other characteristics of the fibercould have been applied. The numerical aperture NA of the multimodefiber piece 3 can, for example, be equal to or greater than 0.50, forexample, NA 0.66. A diameter d of a core of the multimode fiber piece 3can, for example, furthermore fulfill the relation d 80 μm, for example,d 100 μm.

The mechanical mount 5 of the key holder 1 will be further describedbelow. The mechanical mount 5 in principle comprises several differentfunctional parts. It basically comprises a ferrule mount 11 that isbasically provided in the form of a hollow cylinder, wherein an end partof the ferrule 2 is provided inside the hollow cylinder 11 and whereinat least a part of the optical key 4 (here: the entire optical key 4) isalso provided inside the hollow cylinder 11. The end part of the ferrule2 and the part of the optical key 4 are connected to the ferrule mount11, respectively. In the depicted embodiment, both parts are glued tothe ferrule mount 11. In the present embodiment, the ferrule 2 is gluedto the ferrule mount 11 exclusively using epoxy. The optical key 4comprises a material mixture of epoxy and scattering particles, thematerial of the optical key 4 can therefore also be used for gluingpurposes. Other materials or methods can, however, be used forconnecting the ferrule 2 and the optical key 4 to the ferrule mount 11.If the optical key 4 is provided fully or at least partly inside theferrule mount 11, the optical key 4 is well protected against mechanicaldamage like scratches etc. It is noted that mechanical damage and inparticular scratches can alter the response of the optical key 4 whenprobed with an optical challenge which can irreversibly destroy theoptical key 4.

The ferrule mount 11 is provided inside a casing 12 of basicallycylindrical shape. A spring mechanism comprising a spring 10 isfurthermore coupled to the ferrule mount 11 and the casing 12. Thecoupling contact occurs in the areas depicted with reference signs 13and 14 shown in the drawings as an example. Due to the spring mechanism,the ferrule mount 11 is therefore pushed towards the mechanical mountterminator 50 when the mechanical mount 5 is mounted to the mechanicalmount terminator 50.

The mechanical mount 5 furthermore comprises a rotation stop mechanismfor enhancing the positional accuracy when connecting the mechanicalmount 5 to the mechanical mount terminator 50. In the depictedembodiment, the mechanical mount 5 therefore comprises a guiding ring 9which is provided coaxially with ferrule 2 and is arranged outside theferrule 2. The guiding ring 9 is adapted to be slidably guided inside oroutside (here: inside) a corresponding terminator guiding ring 59provided at the mechanical mount fiber terminator 50. The guiding ring 9comprises a protruding pin 8 for securing the mechanical mount 5 againstrotation in the mechanical mount terminator 50. It is also possible,however, to provide two or more pins for realizing a rotational lock.

The mechanical mount 5 of the key holder 1 additionally comprises aring-shaped coupling nut 15 with a thread 16. The coupling nut 15comprises a recess 18 into which an end part of the guiding ring 9 ispositioned. The coupling nut 15 can in this way be rotated around theinner parts and in particular around the guiding ring 9 of the keyholder. The guiding ring 9 is mechanically coupled to the ferrule 2comprising the multimode fiber piece 3 via the ferrule mount 11. It istherefore possible that the orientation of the multimode fiber piece 3stays the same when the mechanical mount 5 is connected to themechanical mount terminator 50 via the depicted screw mechanism. Themechanical mount 5 could alternatively be connected to the mechanicalmount terminator 50 by a screw mechanism or via a magnetic mechanism.

A ring 17 is provided inside a recess 19 on the end part of the guidingring 9. The recess 19 is there to mount ring 17. The ring 19 preventsthe ferrule mount 11 from falling out of the casing 12.

The mechanical mount terminator 50 shall be further described below. Themechanical mount terminator 50 comprises a base plate 51 having athrough-hole 52 facing the front face 6 of the multimode fiber piece 3when the mechanical mount 5 is mounted to the mechanical mountterminator 50. The base plate 51 further comprises a ring-shapedreception part 53 which is coaxially arranged with the optical axis A ofthe system. The reception part 53 is adapted for receiving at least partof the ferrule 2 and its inner diameter is adapted to fit to the outerdiameter of the ferrule. In the depicted embodiment, the ferrule 22comprises a tapered part and an end part of the ring-shaped receptionpart 53 also comprises a respectively tapered part.

The base plate 51 further comprises a ring-shaped fixation part 59 whichalso functions as a terminator guiding ring 59. This ring-shapedfixation part 59 is coaxially arranged with respect to the ring-shapedreception part 53 and is arranged radially outside and with a radialdistance to the reception part 53. The ring-shaped fixation part 59comprises a bore hole 58 for receiving the pin 8 of the mechanical mount5 of the key holder 1 to secure the mechanical mount 5 and themechanical mount terminator 50 against relative rotation. A thread 60 isprovided on the outer face 62 of the terminator guiding ring 59. Inother words, the terminator guiding ring 59 is adapted to provide twodifferent functions. On its outer face, it provides a coupling mechanismin terms of the threads 60, on its inner face 61, it provides a guidingfunction for correctly positioning the guiding ring 9. The innerdiameter of the terminator guiding ring 59 is adapted to fit to theouter diameter of the guiding ring 9 of the mechanical mount 5. They canbe in sliding contact.

An objective 105 is also depicted in FIG. 2 . The objective 105 focuseslight onto the front facet 6 of the multimode fiber piece 3 when the keyholder 1 is mounted to the mechanical mount terminator 51. The objective105 and the mechanical mount terminator 50 are arranged in a fixeddistance and a fixed orientation with respect to one another. Thisprovides a higher accuracy in authenticating procedures.

FIG. 3 schematically illustrates a sectional view of the system shown inFIG. 2 when the key holder 1 with the optical key 4 is mounted. Samereference signs thereby indicate the same features. The mechanical mount5 is connected to the mechanical mount terminator 50 by a screwmechanism. FIG. 3 shows that the thread 16 of the coupling nut 15engages with the thread 60 of the terminator guiding ring 59 of themechanical mount terminator 50. The guiding ring 9 is furthermore insliding contact with the inner face 61 of the terminator guiding ring59. The pin 8 is furthermore guided inside the bore hole 58 provided inthe terminator guiding ring 59.

The ferrule 2 is inserted into the ring-shaped reception part 53radially inside with respect to the fixation mechanism and rotation stopmechanism. The outer face 21 of the ferrule is in sliding contact withthe inner face 63 of the terminator guiding ring 59. The tapered section22 of the ferrule 2 is furthermore in contact with the respectivetapered section provided in the ring-shaped reception part 53. Thespring 10 pushes the multimode fiber piece 3 provided inside the ferrule2 into the direction of the objective 105.

FIG. 4 schematically illustrates a second embodiment of the presentinvention with a different optical key 4. The following will concentrateon the differences between the second embodiment and the firstembodiment. Apart from these differences, the first embodiment and thesecond embodiment are identical. Whereas in FIG. 3 the optical keycomprises a material mixture of epoxy and scattering particles, theoptical key depicted in FIG. 4 is provided as a plug. In the depictedsecond embodiment, the plug comprises a ceramic. It can be a glassceramic plug which has advantageous material properties. It can inparticular have a thermal reliability parameter which is lower than1/(200,000 Kelvin) where the thermal reliability parameter is theabsolute value of one or more summed up temperature coefficients of theoptical key 4, and where the thermal reliability parameter is inparticular the temperature coefficient of the optical path length {1/S}{ds/dT}. For further details, reference is made to EP 3 252 740 B1 whichdescribes further details about the material properties of the opticalkey 4.

In the depicted embodiment, the ceramic plug 4 comprises a bore holeinto which the end part with the back facet 7 of the multimode fiber 3is inserted. The scattering plug 4 can, for example, be optically gluedto the end of the fiber 3. The plug 4 could in addition also beconnected or glued to the ferrule mount 11. The plug 4 couldalternatively be only in contact with the back facet 7 of the multimodefiber piece 3 without embedding the end part of the multimode fiberpiece 3.

FIG. 5 schematically illustrates a third embodiment of the presentinvention wherein the position of the optical key 4 differs from theposition of the optical key 4 in the first and second embodiment. Thedifferences between the third embodiment and the first and secondembodiments will be described below. Apart from these differences, thethird embodiment and the first and the second embodiment can beidentical. According to the third embodiment, the multimode fiber piece3 is shorter. Here, as a consequence, the entire multimode fiber 3 isprovided outside the ferrule mount 11 and outside the casing 12, thesame holds here for the optical key 4. The material of the optical key 4can, for example, be chosen to be very hard and durable since it isprovided in the protruding part of the ferrule 2. It can, for example,be the glass ceramic material as applied according to the secondembodiment of the present invention.

Concerning the mounting of the ferrule 2, it is noted that a part of theferrule 2 provided inside the ferrule mount 11 is also in contact withthe ferrule mount 11. The mounting region of the ferrule 2 is thereforebigger than the mounting region of the ferrule 2 in the first and secondembodiment. The mounting region of the ferrule 2 can, however, also bedesigned as in the first and second embodiment, the mounting region thushaving the same length as in the first and second embodiment.

As shown in FIG. 5 , the ferrule 2 can furthermore be subdivided intothree parts 2 a, 2 b, and 2 c. The multimode fiber is provided insidethe part 2 a. The optical key 4 is provided inside the part 2 b. Anothermaterial 24 can be provided for filling or stabilizing purposes insidethe part 2 c. It is not necessary that this part has any specificoptical properties. The inside of part 2 c can alternatively stay partlyor completely empty, as long as the ferrule 2 as such has the necessarystiffness or form stability.

FIG. 6 schematically illustrates a fourth embodiment of the presentinvention wherein the position of the optical key 4 differs from theposition of the optical key 4 in the third embodiment. The differencesbetween the fourth embodiment and the third embodiment will be describedbelow. Apart from these differences, the fourth embodiment and the thirdembodiment can be identical. As shown in FIG. 6 , the optical key 4 isprovided inside the ferrule 2 at the very beginning of the ferrule 2.The optical key 4 comprises a front facet 25 through which light canenter directly into the optical key 4 without the separate provision ofa multimode fiber piece 3. Stated differently, the optical key 4 and theoptical multimode fiber piece 3 are provided as one and the sameelement.

It is once again stressed that the multimode fiber piece 3 is only anexample for a multimode light guide 3. In all embodiments depicted inthe drawings, the multimode fiber piece can also be changed to anothertype of multimode light guide. A cross section of the multimode lightguide 3 can, for example, be circular, however, other cross-sectionalshapes, for example, a rectangular cross-sectional shape, are alsopossible.

The ferrule 2 is furthermore depicted to have an elongated rotationallysymmetric configuration on the inside and on the outside. This is not,however, necessarily the case. It is also possible that the ferrule 2has a rectangular inner and/or outer cross-sectional shape. An outercross-sectional shape that is not rotationally symmetric can furthercontribute to a rotation stop mechanism.

In the depicted embodiments, a part of the casing 12 oriented oppositethe mechanical mount terminator 50 comprises an opening 26. This can beprovided for ease of manufacturing of the key holder 1, moreparticularly for inserting the optical key 4 inside the ferrule mount11. This opening 26 can, however, also be permanently closed afterwardsto further protect the optical key 4 from scratches or other damages.The opening 26 does not exist at all in other embodiment variants.

FIG. 7 schematically illustrates a system 1000 for authenticating anoptical key 4 by verifying a match of a challenge-response pairaccording to the present invention. The system differs from the systemaccording to the state of the art as depicted in FIG. 1 in that thekinematic base 100 is exchanged against the system 80 comprising theoptical key holder 1 with the mechanical mount 5 and the mechanicalmount terminator 50 according to the present invention. The system 80can in particular be realized according to any one of the embodiments ofthe key holder 1 and the mechanical mount terminator 50 as describedabove. It is noted that any other system according to the state of theart for authenticating an optical key 4 by verifying a match of achallenge-response pair can also be combined with the new key holder 1and the new system 80. In each case, the system 1000 for authenticatingan optical key 4 by verifying a match of the challenge-response paircomprises a challenge forming device 108 for forming a challenge, anoptical key 4 which comprises a scattering material and is receptive tothe challenge formed by the challenge forming device 108, and a responseverifying device 114 which is receptive to the response provided by theoptical key 4 for verifying if the response provided by the optical key4 matches the challenge formed by the challenge forming device 108.

The embodiments depicted in the drawings are not meant to be limiting tothe present invention. The drawings only depict possible realizations ofthe present invention.

Additional examples for embodiments are described below.

Example 1

Key holder (1) comprising:

a ferrule (2);

an optical key (4) comprising a light scattering material, wherein theoptical key (4) is embedded in the ferrule (2) and is provided at afront end of the ferrule (2); and

a mechanical mount (5) mounting the ferrule (2) with the optical key(4),

wherein the optical key (4) comprises a front facet (25) through thatlight can enter into the optical key (4), and wherein the optical key(4) is adapted so that entered light can then be scattered inside theoptical key (4) and can exit the optical key (4) via the front facet(25) of the optical key (4) again, and

wherein the mechanical mount (5) is adapted to be detachably connectedto a mechanical mount terminator (50), wherein the front facet (25) ofthe optical key (4) is oriented in the direction of the mechanical mountterminator (50).

Example 2

Key holder (1) according to Example 1, wherein the following relationholds for an inner diameter df of the ferrule (2): df≥80 μm, inparticular df≥100 μm.

Example 3

Key holder (1) according to any one of the preceding Examples, whereinthe mechanical mount (5) is adapted to be connected to a mechanicalmount terminator (50) by a click mechanism or a screw mechanism.

Example 4

Key holder (1) according to any one of the preceding Examples, whereinthe mechanical mount (5) comprises a rotation stop mechanism (8, 9) forenhancing the positional accuracy when connecting the mechanical mount(5) to a mechanical mount terminator (50).

Example 5

Key holder (1) according to the preceding Example, wherein the rotationstop mechanism (8, 9) comprises a guiding ring (9) coaxially providedwith the ferrule (2) and outside the ferrule (2), the guiding ring (9)being adapted to be slidably guided inside or outside a correspondingterminator guiding ring (59) provided at a mechanical mount terminator(50), the guiding ring (9) comprising at least one radially protrudingpin (8) for securing the mechanical mount (5) against rotation in themechanical mount terminator (50).

Example 6

Key holder (1) according to any one of the preceding Examples, whereinthe mechanical mount (5) comprises a push mechanism (10), for example, aspring mechanism, for pushing the ferrule (2) with the optical key (4)towards a mechanical mount terminator (50) when connecting themechanical mount (5) to a mechanical mount terminator (50).

Example 7

Key holder (1) according to the previous Example, the mechanical mountfurther comprising a ferrule mount (11) that is basically provided inthe form of a hollow cylinder (11), wherein an end part of the ferrule(2) is provided inside the hollow cylinder (11), the end part of theferrule (2) being connected to the ferrule mount (11).

Example 8

Key holder (1) according to the preceding Example, wherein the ferrulemount (11) is provided inside a casing (12) of basically cylindricalshape, wherein a spring mechanism (10) coupled to the ferrule mount (11)and wherein the casing (12) is provided inside the casing (12) to pushthe ferrule mount (11) towards a mechanical mount terminator (50).

Example 9

Key holder (1) according to any one of the preceding Examples,

wherein a spatial positioning accuracy of the front facet (25) of theoptical key (4) with respect to a mechanical mount terminator (50) isequal to or better than 1 μm in each of the three spatial directionsand/or

wherein an angular positioning accuracy of the front facet (25) of theoptical key (4) with respect to a mechanical mount terminator (50) isequal to or better than 1 degree in each of the three angulardirections.

Example 10

Key holder (1) according to any one of Examples 1 to 9, wherein theoptical key (4) comprises a material mixture of epoxy and scatteringparticles, in particular a mixture of epoxy and zinc oxide powder.

Example 11

Key holder (1) according to any one of Examples 1 to 9, wherein theoptical key (4) is provided as a plug, in particular a plug comprising aceramic.

Example 12

A mechanical mount terminator (50) adapted to fit to the mechanicalmount (5) of a key holder (1) as described in any one of Examples 1 to11.

Example 13

A system (80) comprising:

the key holder (1) as described in any one of Examples 1 to 11comprising the mechanical mount (5); and

a mechanical mount terminator (50),

wherein the mechanical mount (5) is detachably mountable or detachablymounted to the mechanical mount terminator (50).

Example 14

The system (80) according to Example 13, wherein the accuracy with whichthe key holder (1) is detachably mounted to the mechanical mountterminator (50) is equal to or better than 10⁻⁶ m.

Example 15

The system (80) as described in any one of Examples 13 to 14, furthercomprising an objective (105), wherein the mechanical mount terminator(50) is fixedly arranged with respect to the objective (105).

Example 16

The system (80) as described in any one of Examples 13 to 15, whereinthe mechanical mount terminator (50) comprises a base plate (51) havinga through hole (52) facing the front face (6) of the optical key (4)when the mechanical mount (5) is mounted to the mechanical mountterminator (5).

Example 17

The system (80) as described in Example 16, wherein the base plate (51)further comprises a ring-shaped reception part (53) coaxially arrangedwith the optical axis (A) of the system (80), the reception part (53)being adapted for receiving at least part of the ferrule (2).

Example 18

The system (80) as described in Example 17, wherein the base plate (51)further comprises a ring-shaped fixation part (59) being coaxiallyarranged with respect to the ring-shaped reception part (53) and beingarranged radially outside and in particular with a radial distance tothe reception part (53), the ring-shaped fixation part (59) comprising abore hole (58) for receiving a pin (8) of the mechanical mount (5) inorder to secure the mechanical mount (5) and the mechanical mountterminator (50) against relative rotation.

Example 19

A system (1000) for authenticating an optical key by verifying a matchof a challenge-response pair, comprising:

a challenge forming device (108) for forming a challenge;

an optical key (4) which comprises a scattering material and isreceptive to the challenge formed by the challenge forming device (108);and

a response verifying device (114) which is receptive to the responseprovided by the optical key for verifying if the response provided bythe optical key (4) matches the challenge formed by the challengeforming device (108),

characterized by the system (80) as described in any one of Examples 13to 18, wherein the optical key (4) is held by the key holder (1).

Example 20

System according to Example 19, wherein the following relation holds:

q=Int_(exit)/Int_(enter)≥50%, in particular q≥70%,

wherein Int_(enter) is the intensity of light entering the optical key(4) via its front facet (25) and Int_(exit) is the intensity of lightexiting the optical key (4) via its front facet (25) after having beenscattered by the light scattering material of the optical key (4).

The present invention is not limited to embodiments described herein;reference should be had to the appended claims.

LIST OF REFERENCE SIGNS

-   -   1 key holder    -   2 ferrule    -   2 a-c parts of ferrule    -   3 multimode fiber piece    -   4 optical key    -   5 mechanical mount    -   6 front facet    -   7 back facet    -   8 pin, rotation stop mechanism    -   9 guiding ring, rotation stop mechanism    -   10 spring, push mechanism    -   11 ferrule mount, hollow cylinder    -   12 casing, cylindrical shape    -   13 contact area for spring    -   14 contact area for spring    -   15 coupling nut    -   16 thread    -   17 ring    -   18 recess    -   19 recess    -   20 opening    -   21 outer face of ferrule    -   22 tapered part    -   23 outer face of guiding ring    -   24 material or space inside part 2 c of the ferrule    -   25 front facet of the optical key    -   26 opening    -   50 mechanical mount terminator    -   51 base plate    -   52 through hole in the base plate    -   53 ring-shaped reception part    -   58 bore hole for receiving a pin    -   59 ring-shaped fixation part, terminator guiding ring    -   60 thread    -   61 inner face of the terminator guiding ring    -   62 outer face of the terminator guiding ring    -   63 inner face of the terminator guiding ring    -   64 contact area    -   80 system    -   100 kinematic base    -   101 top plate (key holder)    -   102 bottom plate    -   103 optical key    -   104 structure for positioning    -   105 objective    -   106 laser    -   107 half-wave plate    -   108 DMD (digital micromirror device)    -   109 lens    -   110 aperture    -   111 beam splitter    -   112 lens    -   113 partial beam splitter    -   114 camera, CCD    -   115 direction of propagating light    -   1000 QSA demonstrator set-up    -   A optical axis

What is claimed is: 1-23. (canceled) 24: A key holder comprising: aferrule; a multimode light guide at least partly embedded inside theferrule, the multimode light guide comprising a front facet and a backfacet which are arranged at opposite ends of the multimode light guide;an optical key which comprises a light scattering material; and amechanical mount which is configured to mount each of the ferrule, themultimode light guide, and the optical key, wherein, the back facet ofthe multimode light guide contacts the optical key, the multimode lightguide is configured so that light can enter into the multimode lightguide via the front facet, propagate through the multimode light guide,be scattered by the optical key, and propagate back through themultimode light guide and exit via the front facet, the mechanical mountis further configured to be detachably connected to a mechanical mountterminator, and the front facet of the multimode light guide is orientedin a direction of the mechanical mount terminator. 25: The key holder asrecited in claim 24, wherein, the multimode light guide is furtherconfigured so that light which can enter into the multimode light guidevia the front facet propagates through the multimode light guide via atotal internal reflection, and the multimode light guide is a multimodefiber piece which has a numerical aperture NA with a relation ofNA≥0.50. 26: The key holder as recited in claim 24, wherein, themultimode light guide further comprises a core having a diameter d, andthe diameter of the core is ≥80 μm. 27: The key holder as recited inclaim 24, wherein the mechanical mount is further configured to beconnected to the mechanical mount terminator via a click mechanism or ascrew mechanism. 28: The key holder as recited in claim 24, wherein themechanical mount comprises a rotation stop mechanism which is configuredto enhance a positional accuracy when connecting the mechanical mount tothe mechanical mount terminator. 29: The key holder as recited in claim28, wherein, the mechanical mount terminator comprises a terminatorguiding ring, the rotation stop mechanism comprises a guiding ring whichis arranged coaxially with the ferrule and outside the ferrule, theguiding ring being configured to be slidably guided inside or outsidethe terminator guiding ring, and the guiding ring comprises at least oneradially protruding pin for securing the mechanical mount against arotation in the mechanical mount terminator. 30: The key holder asrecited in claim 24, wherein. the mechanical mount comprises a pushmechanism which is configured to push the multimode light guide towardsthe mechanical mount terminator when connecting the mechanical mount tothe mechanical mount terminator, and the push mechanism is a springmechanism. 31: The key holder as recited in claim 30, wherein, themechanical mount further comprises a ferrule mount which comprises ahollow cylinder, the ferrule comprises an end part which is arrangedinside of the hollow cylinder, at least a part of the optical key isarranged inside the hollow cylinder, and the end part of the ferrule andthe part of the optical key are each connected to the ferrule mount. 32:The key holder as recited in claim 31, further comprising: a casinghaving a substantially cylindrical shape; and a spring mechanism,wherein, the ferrule mount is provided inside the casing, and the springmechanism is coupled to the ferrule mount and is arranged inside thecasing, the spring mechanism being configured to push the ferrule mounttowards the mechanical mount terminator. 33: The key holder as recitedin claim 24, wherein the optical key is completely arranged inside theferrule. 34: The key holder as recited in claim 24, wherein at least oneof, a spatial positioning accuracy of the front facet of the multimodelight guide with respect to the mechanical mount terminator is equal toor better than 1 μm in each of three spatial directions, and an angularpositioning accuracy of the front facet of the multimode light guidewith respect to the mechanical mount terminator is equal to or betterthan 1 degree in each of the three angular directions. 35: The keyholder as recited in claim 24, wherein, the optical key comprises, asthe light scattering material, a material mixture of epoxy andscattering particles, or the optical key is provided as a plug. 36: Thekey holder as recited in claim 24, wherein the multimode optical lightguide is a multimode fiber piece. 37: A mechanical mount terminatorwhich is configured to fit to the mechanical mount of the key holder asrecited in claim
 24. 38: A system comprising: the key holder as recitedin claim 24; and a mechanical mount terminator, wherein, the mechanicalmount of the key holder is configured to be detachably mountable ordetachably mounted to the mechanical mount terminator. 39: The system asrecited in claim 38, wherein an accuracy with which the key holder isdetachably mountable to the mechanical mount terminator is equal to orbetter than 10⁻⁶ m. 40: The system as recited in claim 38, furthercomprising: an objective, wherein, the mechanical mount terminator isfixedly arranged with respect to the objective. 41: The system asrecited in claim 38, wherein the mechanical mount terminator comprises abase plate having a through hole which faces the front face of themultimode light guide when the mechanical mount is mounted to themechanical mount terminator. 42: The system as recited in claim 41,wherein, the base plate further comprises a ring-shaped reception partwhich is arranged coaxially with an optical axis of the system, and thering-shaped reception part is configured to receive at least a part ofthe ferrule. 43: The system as recited in claim 42, wherein, themechanical mount of the key holder comprises a pin, the base platefurther comprises a ring-shaped fixation part which is arrangedcoaxially with respect to the ring-shaped reception part and which isarranged radially outside and with a radial distance to the ring-shapedreception part, and the ring-shaped fixation part comprises a bore holefor receiving the pin of the mechanical mount in order to secure themechanical mount and the mechanical mount terminator against a relativerotation. 44: A system for authenticating an optical key by verifying amatch of a challenge-response pair, the system comprising: a challengeforming device which is configured to form a challenge; an optical keywhich comprises a scattering material, the optical key being receptiveto the challenge formed by the challenge forming device by providing aresponse; and a response verifying device which is receptive to theresponse provided by the optical key for verifying if the responseprovided by the optical key matches the challenge formed by thechallenge forming device, wherein, the optical key is provided by thesystem as recited in claim 38, and the optical key is held by the keyholder of the system as recited in claim
 38. 45: The system according toclaim 44, wherein the following relation holds:q=Int_(exit)/Int_(enter)≥50%, wherein, Int_(enter) is an intensity oflight entering the multimode light guide via the front facet, andInt_(exit) is an intensity of light exiting the multimode light guidevia the front facet after having been scattered by the light scatteringmaterial of the optical key.